We now know that anatomically modern humans with interfertile with Neanderthals and Denisovans. It seems likely that AMH was interfertile with a very divergent African group whose genes show up today in Bushmen and Pygmies. Almost certainly, Neanderthals and Denisovans were interfertile.

Of course all of this was overwhelmingly likely even before we found genetic evidence. Mammalian lineages with such time-depths of separation are almost always interfertile.

Judging from these facts, all these groups – and more – would be members of the same biological species, in Ernst Mayr’s definition. What can we conclude from this?

Nothing. We know that the reproductive systems of these groups are compatible, but that’s what we started with. Can we conclude that Neanderthal behavior was really more like ours than the archaeological record suggests, because of this biological compatibility? No. Can we conclude that Denisovans had technological capabilities other than those shown in known East Asian artifacts? Nope.

What can we conclude from this? Nothing. We know that the reproductive systems of these groups are compatible, but that’s what we started with.

What we can conclude is that with very high likelihood the other guys also had 23 distinct chromosomes. Because that’s a major determinant of viability/fertility. The point of the chromosome fusion (24 –> 23) is likely to be a point where new Mayr-style species was born.